Non-linear electric loads such as variable frequency motor drives with diode rectifier inputs draw current from an AC sinusoidal voltage source at multiple frequencies, that is, at the fundamental frequency of the AC source, and also at harmonic multiples of the fundamental frequency. The currents drawn at harmonic frequencies are undesirable, and considered distortion. Passive filters are commonly used to reduce the amount of undesirable harmonic current flowing from the load into the AC source. As an example, an input passive filter placed between the AC source and the harmonic load, as shown in FIG. 1, will reduce the harmonic currents flowing in the source (Iin) as compared to the harmonic load (Iout).
The purpose of the input passive filter is to reduce the amount of harmonic current flowing in the source. This is primarily accomplished by providing a trap circuit (L2/C2 in FIG. 2) that has low impedance at the relevant harmonic frequency, thus attracting the load harmonic currents to flow in the trap circuit and not in the source. The line reactor L1 further impedes the harmonic currents from flowing in the source. The trap circuit is connected to the line reactor tap terminal T3 for optimal harmonic performance.
Another passive filter implementation is shown in FIG. 3, wherein the line reactor L1 does not have a tap terminal, and the trap is connected to the line reactor end terminal T2. Under ideal electrical system conditions, with a sinusoidal ac voltage source, the passive filter circuit topology shown in FIG. 2 is a higher performing filter (achieves lower source current harmonic levels) for variable speed motor drive loads than the filter shown in FIG. 3.
Under non-ideal and real world conditions, the AC voltage source is not purely sinusoidal, sometimes containing up to 1%-5% background distortion (1%-5% rms voltage at harmonic frequencies of the fundamental) or more. In remote locations, such as rural oil fields, the voltage source can experience voltage distortion exceeding 10%. Background voltage distortion is defined as the distortion in the voltage provided by the electric utility or generator. It is equivalent to the distortion level of the input AC voltage with loads not operating or drawing current. As loads are added to the power system within a facility, additional voltage distortion will be produced by their harmonic load currents. The presence and level of background voltage distortion can significantly lower the performance of some passive filters, depending on the filter topology. As shown by the graph in FIG. 4, the tapped line reactor filter shown in FIG. 2 outperforms the non-tapped line reactor filter shown in FIG. 3 at low input background voltage distortion. However, at higher background voltage distortion, such as distortion greater than 2% vTHD (voltage total harmonic distortion), the non-tapped line reactor filter shown in FIG. 3 outperforms the tapped filter shown in FIG. 2 and produces a lower current THD.
It is known in the field that connecting the trap to the tapped terminal T3 as shown in FIG. 2 will improve the performance of the filter under normal voltage conditions (vTHD<1%), compared to connecting the trap the end terminal T2 as shown in FIG. 3. Passive filter topologies can be optimally built for low input voltage distortion (FIG. 2) or for high input voltage distortion (FIG. 3).
As the background AC voltage distortion varies over time at any given location on the AC network, a passive filter that provides optimal performance under any input distortion condition is needed.